A gyrator is an electric two-port network element in which the voltage at the output is proportional to the current at the input, the sign of the voltage changing when the input and output are reversed, if the current is an alternating current, the same is thus converted into either an in-phase alternating voltage or an alternating voltage in phase opposition, depending on the port of the gyrator to which this current is applied. The gyrator is necessary as a fifth linear element, in addition to the resistor, capacitor, inductor and ideal transformer, to realize two-port network elements that allow an alternating voltage to pass only in one direction (isolators), or three-port or multi-port elements that allow an alternating voltage to pass a port only to a next port in a fixed rotational direction (circulators).
So as to render the conversion of the input current into the output voltage dependent on the direction of current through the gyrator, Faraday rotation in ferrites under the influence of an external magnetic field is used in the microwave range. For this purpose, it is necessary for an electromagnetic wave generated by the input current to propagate in the ferrite. The dimensions of the ferrite must therefore be in the order of magnitude of the wavelength, bringing the ferrite to an unpracticable size for frequencies in the radio or audio frequency range. This also ceases to operate efficiently at frequencies below the microwave range. Additionally, as a result of the physical dimensions, every gyrator is limited to a substantially narrow frequency band.
As an alternative, a gyrator may also be realized as an active circuit composed of transistors and fed-back operational amplifiers. Such a circuit, however, requires a power supply unit and produces both noise and heat.
A passive gyrator for lower frequencies is known from U.S. Pat. No. 2,649,574, in which the Faraday rotation in the ferrite is replaced with a planar Hall effect. The disadvantage is that high contact resistances impede both coupling of the current into the Hall effect material and tapping of the Hall voltage, which impairs the efficiency of the gyrator.
It is therefore the object of the invention to provide a gyrator which, at low frequencies in the order of magnitude of 1 to 100 MHz, operates more efficiently than gyrators according to the prior art.
This object is achieved according to the invention by a gyrator according to the main claim. Further advantageous embodiments will be apparent from the dependent claims.